Ultra-thin and high selective emission by combining lossless layer

Abstract

The growing awareness of the energy depletion and environmental concerns has motivated the improving the efficiency of existing cooling technologies and pursuing new alternative cooling systems. Radiative cooling is an appealing concept for the 21st century due to its passive way to cooling and environmental sustainability. Fundamentally, radiative cooling is based on high solar reflection and infrared (IR) emission. For more efficient passive cooling, selective IR emitters (SE) are essential because most of the terrestrial thermal radiation at ambient temperature (i.e., 300K) can propagate through the atmospheric transparent spectral window (i.e., 8-13 μm) to ultra-cold heat sink (i.e., universe; 3K). Research in recent decades has yielded a variety of passive selective emitter designs comprising sophisticated emissive coatings such as photonic structures, meta-materials, and multi-stacking (>5 layers) structures. Although efficient, these designs are costly and demands complicated fabrication technology which can restrict the mass-production. Here, we propose the ultra-thin/near-unity selective emitter (UNSE) which does not need any complicated technology such as photolithography process. Overall layouts of UNSE consists of high index lossless layer (HILL; Ge) and IR lossy layers (SiO2 and Si3N4). By combining a HILL with a highly absorbent dielectric layers, IR emission can be remarkably enhanced due to the light trapping at lossy layers. Thus, UNSE can consist of various materials which satisfy the optical features such as HILL or IR lossy layers. Furthermore, we achieved sub-ambient cooling in daytime, by laminating the solar opaque/IR transparent layer (SOIT), which can be self-assembled on the UNSE. These successful demonstrations prove that our proposed structure can be applied to lower the temperatures of heat-sensitive and high-power electronics with simple fabrication method.